Effect of dissolved oxygen and hydrogen ion on corrosion rate and passivation of carbon steel boiler tube

Abstract

The corrosion rate of carbon steel was characterized at various concentrations of dissolved oxygen and pH by potentiodynamic method. In pH 7, the corrosion rate was found to be minimum at 2 ppm O2. This minimum was likely due to a difficulty of passive film formation below 2 ppm O 2; oxygen reduction is enhanced above 2 ppm O2. In pH 10, corrosion rate was found to be low when the oxygen concentration was below 2 ppm O2, a result of stabilization of passive film which acted as a barrier to O2 diffusion toward the metal surface. The pH effect was also investigated by incorporating HCl, NaOH, NH4OH, Na 3PO4, or their combination. For below pH 5, the corrosion rate was found to increase resulting from sufficient hydrogen and chloride ions. For pH between 5 and 10, the corrosion rate was found to be low and constant due to the barrier (passive film) of O2 diffusion. For pH above 10, as passive film was more stable, the corrosion rate was found to be lower.